The invention relates generally to x-ray tubes and, more particularly, to an antiwetting coating for a liquid metal bearing in an x-ray tube and a method of making same.
X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is typically necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearing assembly, which typically includes tool steel ball bearings and tool steel raceways positioned within the vacuum region, thereby requiring lubrication by a solid lubricant such as silver. Wear of the silver and loss thereof from the bearing contact region increases acoustic noise and slows the rotor during operation.
In addition, the operating conditions of newer generation x-ray tubes have become increasingly aggressive in terms of stresses because of G forces imposed by higher gantry speeds and higher anode run speeds. As a result, there is greater emphasis in finding bearing solutions for improved performance under the more stringent operating conditions.
A liquid metal bearing (i.e. a spiral groove bearing, or SGB) may be employed in lieu of ball bearings. Advantages of liquid metal bearings include a high load capability and a high heat transfer capability due to an increased amount of contact area as compared to a ball bearing. Advantages also include low acoustic noise operation. Gallium, indium, or tin alloys are typically used as the liquid metal, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube.
Liquid metal bearings are typically fabricated having a small gap, up to a few hundred microns, formed between stationary components and rotating components. The liquid metal is positioned in the small gap and prevents direct metal-to-metal contact from occurring. Liquid metals typically used in an SGB tend to be highly reactive and corrosive. The liquid metal of an SGB may react with a base metal that it contacts, thus consuming the liquid metal and shortening the life of the SGB. The liquid metal is also prone to migration within the bearing and can readily migrate from its operating location in the small gap. If liquid metal migration is unchecked the SGB can become starved of liquid metal, which can lead to metal-to-metal contact between rotating and stationary components, causing early life failure.
As such, an SGB typically includes an antiwetting coating that is positioned on components to avoid liquid metal migration within the SGB. The antiwetting coating typically is a structure or compound that repels the liquid metal and prevents creepage and migration of the liquid metal. That is, the antiwetting coating prevents migration from the location within the SGB where it serves to maintain a separation between the stationary and rotating components. Known antiwetting coatings include TixOy and Al2O3, which are more stable compounds than the liquid metals typically used in an SGB and are thus not prone to degradation due to contact with the liquid metal. However, TixOy and Al2O3 are relatively soft materials that are vulnerable to damage during processing. If damage in the coating occurs the parts are typically re-processed, adding cost and time to the manufacturing process. If the damage is not detected it can lead to early life failure of the bearing.
Another known antiwetting coating includes TiN, which has a significantly greater hardness than, for instance, TixOy. The Mohs hardness of TiN is approximately 9 while that of TixOy is approximately 5-6. As such, TiN can provide an effective antiwetting coating that is also scratch resistant and robust. However, TiN is prone to oxidation at elevated temperature, such as above 500° C. This oxidation can occur even during dry hydrogen firing if the dewpoint is not sufficiently low. Volatile Ga2O given off by the bearing has also been shown to cause oxidation of the TiN coated surfaces. As such, conversion of TiN to TixOy can result in a material on the surface of the TiN that has a reduced hardness and drop in scratch resistance. That is, although TiN may be selected as an antiwetting coating because of its high hardness, such benefit can be lost if, during processing, oxidation of the TiN occurs.
Therefore, it would be desirable to design an x-ray tube with an SGB having a robust and high hardness coating that is not susceptible to oxidation.